freezing seeds before germination

Alys Fowler: why seeds need a cold snap

For many seeds, dormancy is broken by a drop in temperature. So you can leave it to nature – or cheat

Hawthorn. Photograph: Alamy

Hawthorn. Photograph: Alamy

Last modified on Fri 1 Dec 2017 15.38 GMT

S eed dormancy sounds rather appealing right now. I’d like to wrap myself up in a warm coat and spend the next few months in stasis until spring arrives. If only that was what really happens. It may seem as if seeds are just lying in the soil waiting for the temperature to rise, but something else is going on.

For many seeds, dormancy is broken not by a rise in temperature but by a drop. It’s the cold of winter these seeds are after. The season’s cycle of frost, harsh winds and bitter rains slowly softens the tough seed coat, rolling it around in the soil, freezing and then thawing again, until the seed can take up water and germinate. This is known as stratification, or cold treatment. You can tell seeds that need a cold period before germinating because they have hard bony coats that are impervious to water.

Having such a tough shell ensures that germination occurs only when conditions are right. Weather fluctuates; you don’t want your seed jumping into germination just because autumn has a few cold nights and then a warm one. It’s not spring yet and those cold nights did not represent winter. So time and temperature are the keys necessary to unlock germination for many seeds. For others it may be light, smoke, certain chemicals or spending time in an animal’s gut.

Many domesticated plants have undergone numerous selection pressures, so dormancy isn’t such a big deal. Wild plants, however, are fiercely dedicated to their dormancy methods. Common plants that require stratification include apples, sloes, hawthorns, plums and acorns, but also smaller seed from herbaceous perennials such as aquilegia, lavender, sage, sedums, perennial sweet peas, wild rose and hops.

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One way to break this dormancy is to leave it up to nature. This is not the fast route, but it’s a sure one. Sow seeds outside now in pots, cover with grit (mostly so that it’s easy to weed out any interlopers) and leave them to the elements. Be patient: signs of life should appear, if not this spring, then the following. You can also make a seed bed for stratification. Use a gritty compost mix, one part grit to three parts compost. Excess moisture can be a problem, so make sure it is free-draining.

Or cheat. You can use the fridge (and sometimes the freezer), placing the seed either on a damp sheet of kitchen towel or in damp vermiculite in a freezer bag (on which you can write all the details: seed source, date, temperature requirements etc). Most seed that requires a winter chill will need between two weeks and three months before dormancy is broken. Keep checking the seeds until you see signs of life. Once they germinate, take them out of the fridge, pot them up and keep them frost-free until you can put them outside.

For many seeds, dormancy is broken by a drop in temperature. So you can leave it to nature – or cheat

How To Cold Stratify Seeds For Spring Planting

Many native varieties, like Prairie Coneflower, require cold stratification if seeding in spring. The good news is this is an easy process for any gardener.

Many wildflowers—especially native varieties—have clever mechanisms in place that help protect them from germinating too early in the spring or too late in the summer. These varieties re-seed naturally in the wild and stay dormant until the proper time for them to start sprouting. More and more gardeners are seeing the benefits of growing native varieties in their landscapes and with a simple technique called cold stratification, you can easily add these wildflowers to your garden in the spring.

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Cold Stratify Seeds: Why Not Just Sow The Seed?

Many annual varieties, like Zinnias, and Sunflowers, have soft shells and can simply be sprinkled on bare soil in the spring. But some perennials, especially native wildflowers, have a hard coating that helps protect the outer shell from breaking and sprouting too early. We’ve all experienced an unseasonably-warm spell in in the middle of January or February — this mechanism helps prevent the seeds from being tricked into coming out of dormancy until it’s just the right time.

The good news for gardeners is that the natural cold stratification needed for germination can be forced with just a few materials, water, a refrigerator, and patience.

If you’re planting native wildflowers or varieties that require cold stratification in the fall, this step isn’t necessary. Nature will do what it does best during the winter months and cold stratify the seeds for you.

There are quite a few native varieties that should be cold stratified before planted in spring. We chose Prairie Violet Seeds, St. John’s Wort, and Tennessee Purple Coneflower as some of our varieties to plant.

Cold Stratify Seeds: Varieties

There are several perennial and native seed varieties need to be manually broken from dormancy in order to sprout and thrive in your garden. If you’re planting native seeds and aren’t sure, chances are you should at least scarify and soak your seeds before planting.

Common varieties that require cold stratification for spring planting:

  • Milkweed (Asclepias)
  • Lupine (Lupinus)
  • St. John’s Wort (Hypericum perforatum)
  • Prairie Coneflower (Ratibida)
  • Prairie Violet (Viola pedatifida)
  • Pincushion Flower (Scabiosa)
  • Marsh Marigold (Caltha palustris)
  • Perennial Sunflowers (Helianthus)
  • Wild Geranium (Geranium maculatum)
  • Rudbeckia (most varieties)
  • Coneflower (some varieties)
  • Soapwort (Saponaria ocymoides)
  • Primrose (Oenothera speciosa)
  • Larkspur (Delphinium)
  • Shooting Star (Dodecatheon meadia)
  • Heliopsis
  • Lavender/Hyssop (Agastache foeniculum)
  • Catmint (nepeta)
  • Ironweed (Vernonia gigantea)
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Although this is a comprehensive list of the most common varieties, there are other seeds that do require cold stratification before spring planting. It’s best to call us at (877) 309-7333 if you aren’t sure.

Most of the materials you need to cold stratify seeds can be found in your home or tool shed.

Cold Stratify Seeds: Gather Materials

Cold stratification is an extremely easy process and once you’ve done it once, you’ll no doubt get the hang of it. The first step is to gather the materials needed, all of which can be found in your home, tool shed, or with a quick trip to the hardware store.

Materials for Cold Stratification:

  • Seeds
  • An all purpose sand mixture and/or Peat Moss
  • Paper Towels
  • Water
  • Plastic ziploc bags
  • A Sharpie or pen for labeling
  • Mixing bowls
  • Refrigerator

Cold Stratify Seeds: Step by Step Process

Now that you have your materials, you can use three different methods for cold stratifying your seeds. All three of these methods work equally well and offer up different ways to basically keep the seeds moist in your refrigerator until it’s time to plant. We’ll go over all three methods:

Learn how to easily cold stratify perennial and native wildflower seeds to help with quick, uniform germination for spring planting.